In the past, there has been known an exhaust purification system of an internal combustion engine which is provided, in the exhaust passage, with a particulate filter which traps particulate matter (below, referred to as “PM”) in exhaust gas and a selective reduction catalyst which reduces and purifies NOX in the exhaust gas by reduction. In such an exhaust purification system, the selective reduction catalyst is provided with an ammonia feed device which feeds urea or ammonia or another ammonia ingredient. In addition, by adjusting the amount of feed of ammonia from the ammonia feed device, the amount of adsorption of the ammonia ingredient at the selective reduction catalyst is maintained at a suitable level. Due to this, the NOX which is contained in the exhaust gas can be reliably removed by the selective reduction catalyst.
In the meantime, PM gradually builds up at the particulate filter along with use of the internal combustion engine. Further, if the amount of buildup of PM becomes greater, the particulate filter will become clogged and the exhaust resistance due to the particulate filter will become greater. For this reason, in an internal combustion engine provided with a particulate filter, when the amount of buildup of PM at the particulate filter becomes a certain extent or more, a filter regeneration processing is performed in which the particulate filter is raised in temperature to remove the PM which has built up at the particulate filter. In an internal combustion engine which is also provided with the above explained selective reduction catalyst in the engine exhaust passage, this filter regeneration processing leads to a rise in temperature of the selective reduction catalyst as well.
However, if the selective reduction catalyst becomes high in temperature, it will no longer be able to adsorb ammonia and accordingly exhaust gas containing ammonia will leak out from the selective reduction catalyst in the phenomenon called “ammonia slip”. If performing the above explained filter regeneration processing, the selective reduction catalyst is raised in temperature, and therefore ammonia slip can occur in this case as well.
Therefore, when performing the filter regeneration processing, it has been proposed to stop the feed of ammonia from the ammonia feed device and to start the filter regeneration processing when the amount of adsorption of ammonia by the selective reduction catalyst becomes a certain value or less (for example, PTL 1). By starting the filter regeneration processing after the amount of adsorption of ammonia by the selective reduction catalyst is decreased in this way, even if the filter regeneration processing leads to a rise in the temperature of the selective reduction catalyst, it is considered possible to suppress the outflow of an ammonia ingredient from the selective reduction catalyst.